Yeast Washing and High Gravity Beer

I read on this page,
http://www.wyeastlab.com/com-yeast-harvest.cfm

Previous Fermentation: Always harvest from a low gravity and low hopped beer. High gravity and/or highly hopped beers can stress the yeast and have detrimental affects on future fermentations. Do not harvest yeast from beers with alcohol contents greater than 6.5% ABV.

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My beers I have been brewing since I started washing yeast have ranged from 4.5%ABV all the way to 10% and lightly to highly hopped and I've harvested yeast from all of them. Because there are varying yeast strains in each batch, I was planning on only reusing them for similar batches, but since they don't really explain why (other than saying it "can stress the yeast and have detrimental *affects", I'd like to know the reasons why and if this really matters as much to a home brewer or this is more of a guideline for a commercial brewer.

That should be *effects, by the way... sorry grammar police are out!

If you think about what the yeast is doing, it makes sense. Let's say you start out with a smack pack. You have some yeast that's as uniform in fermentation characteristics as humanly possible. That yeast will multiply in your beer several times over, creating some amount of mutation.

In a 4.5% beer, most of the yeast survive the fermentation. In higher grav beers, much more of the yeast will die. The yeast that live may be alcohol tolerant, but may not produce the same flavors that the original strain had that made you want them.

This is why pro breweries without labs onsite will pitch fresh yeast every 3-8 batches, of any gravity, and why breweries with yeast labs spend so much time and money watching their yeast for mutation, to make sure they are getting the flavors they want.

The problem with high %ABV beers is that, depending on the alcohol tolerance of that yeast, the beer can kill large portions of the yeast population. What is left will be slightly different than when you started and the subsequent fermentation would not be exactly the same. Hard to say if you could taste the difference though.

I have never seen a good explanation of the mechanism whereby high hop levels inhibit yeast growth. I ignore IBU's when deciding whether to wash the yeast or not.